1. Field of the Invention
The present invention relates to a filter for use in mobile communication apparatuses, such as portable or mobile telephones, that conform to the use for plural high-frequency bands.
2. Description of the Related Art
Hitherto, when two high-frequency bands of mobile communications are relatively adjacent to each other as in a case of a 900-MHz GSM (global system for mobile communications) and a 1.8-GHz DCS (digital cellular system), two mobile communication systems share a single antenna. FIG. 10 is a block diagram showing a structure in which two communication systems share a single antenna. In FIG. 10, 51 refers to an antenna, 52 refers to a duplexer, 53 refers to a GSM-side switch, and 54 refers to a DCS-side switch. The antenna 51 is connected to a first terminal 53a of the GSM-side switch 53 through the duplexer 52, and a GSM transmitter circuit Txgsm and a GSM receiver circuit Rxgsm are connected to a second terminal 53b and a third terminal 53c, respectively. Furthermore, the antenna 51 is connected to a first terminal 54a of the DCS-side switch 54 through the duplexer 52, and a DCS transmitter circuit Txdcs and a DCS receiver circuit Rxdcs are connected to a second terminal 54b and a third terminal 54c, respectively. The frequency band is thus separated for the GSM and the DCS through the duplexer 52, and transmit/receive switching is performed by the GSM-side switch 53 and the DCS-side switch 54. By this arrangement, transmitting-receiving communication between the two mobile communication systems GSM and DCS, sharing a single antenna, can be achieved.
However, in such a conventional structure sharing a single antenna, the antenna is connected to the GSM switch and the DCS switch through the duplexer, and through these switches, the antenna is further connected to the transmitter circuits and the receiver circuits; in which a problem arises with the increased number of constituent members. This problem makes it difficult to reduce the size of the mobile communication apparatus mounting the aforementioned constituent members.